public final class SynonymFilter
extends org.apache.lucene.analysis.TokenFilter

Matches single or multi word synonyms in a token stream.
This token stream cannot properly handle position
increments != 1, ie, you should place this filter before
filtering out stop words.

Note that with the current implementation, parsing is
greedy, so whenever multiple parses would apply, the rule
starting the earliest and parsing the most tokens wins.
For example if you have these rules:

a -> x
a b -> y
b c d -> z

Then input a b c d e parses to y b c
d, ie the 2nd rule "wins" because it started
earliest and matched the most input tokens of other rules
starting at that point.

A future improvement to this filter could allow
non-greedy parsing, such that the 3rd rule would win, and
also separately allow multiple parses, such that all 3
rules would match, perhaps even on a rule by rule
basis.

NOTE: when a match occurs, the output tokens
associated with the matching rule are "stacked" on top of
the input stream (if the rule had
keepOrig=true) and also on top of aother
matched rule's output tokens. This is not a correct
solution, as really the output should be an abitrary
graph/lattice. For example, with the above match, you
would expect an exact PhraseQuery"y b
c" to match the parsed tokens, but it will fail to
do so. This limitations is necessary because Lucene's
TokenStream (and index) cannot yet represent an arbitrary
graph.

NOTE: If multiple incoming tokens arrive on the
same position, only the first token at that position is
used for parsing. Subsequent tokens simply pass through
and are not parsed. A future improvement would be to
allow these tokens to also be matched.